/Engine/Source/Runtime/Engine/Classes/Kismet/KismetMathLibrary.h
Transform a direction vector by the inverse of this matrix - will not take into account translation part. If you want to transform a surface normal (or plane) and correctly account for non-uniform scaling you should use TransformByUsingAdjointT with adjoint of matrix inverse. (Assumes Matrix represents a transform)
static FVector Matrix_InverseTransformVector ( const FMatrix & M, FVector V )